(a) Field of the Invention
The present invention is directed to stabilization of a semi-submersible vessel during outboard handling of loads by cranes on the vessel, the vessel comprising a platform above the water level supported by hollow columns from submersed buoyancy hulls.
(b) Prior Art
Marine structures of this type are less affected by the movement of the sea surface in rough water than vessels floating on the surface and the former have therefore been developed for drilling operation in open sea. However, they present the disadvantage of relatively small stability. Apart from the drilling derrick which is a centrally positioned fixed structure only small cranes for handling light loads can be used on a vessel of this type.
Still it has been proposed in the known art to adapt a vessel of this kind for outboard handling of loads, for instance, by means of a gantry crane mounted near the front of the vessel and to stabilize it by means of water transport to and from ballast tanks in the submersed hulls.
According to this known system a list is imparted to the vessel before hoisting an outboard load opposite the list which is expected to be imparted by hoisting the load.
However, the varying tilts imparted to the vessel during load operation are troublesome for the work and for living on the platform.
Also the maximum admissible preliminary list is limited and therefore only loads up to about 250 tons can be handled and that at a limited distance from the centerline of the vessel. For handling heavier loads the vessel has to be brought in a floating position with the buoyancy hulls at sea level, but there they are exposed to the water movement at the surface and therefore no loads can be handled when the sea is rough.